The development of motors for cars and machine tools, for example, that can be mass produced at low cost and have high torque performance has been desired. In addition, the development of power generators having high efficiency in power generation has also been desired.
In general, the torque performance and the efficiency in power generation of motors depend on the amount of magnetic flux in the air gap between the rotor or mover and the stator, and therefore it is necessary to increase the amount of magnetic flux in the air gap in order to improve the torque performance.
In the following, the torque performance of a motor that is cited as an example is described, but power generators and the like have the same principle torque performance.    It is possible to use magnets having a very strong magnetic force, such as rare earth magnets, in order to increase the amount of magnetic flux in the air gap. However, it is necessary to refrain from using rare earth metals in order to save the rare earth metals resources and lower the cost.    In accordance with another method for improving the torque performance, the air gap (distance between the rotor and the stator) is shortened so as to reduce the magnetic resistance, and thus the amount of magnetic flux is increased. A reluctance motor having an air gap of 0.3 mm has been disclosed as an example (see Japanese Unexamined Patent Publication H11 (1999)-89193).
As described above, the torque performance can be improved by shortening the air gap. In the case where the air gap is shorter than 0.3 mm, the amount of magnetic flux is further increased and the torque performance is further improved. However, it means that the rotor and the stator too close to each other, which increases the risk of the two making contact when the rotor thermally expands. In addition, the shorter the air gap is, the higher the precision is with which the components are positioned at the time of assembly, and therefore it basically ends up costing more for assembly. Thus, there is a practical limit in shortening the gap.
An object of the present invention is to provide a magnetic circuit structure, such as a motor, where the torque performance is improved by increasing the amount of magnetic flux in the air gap while maintaining the air gap at approximately 0.3 mm instead of shortening the air gap more.
Another object of the invention is to provide a magnetic circuit structure having the same or higher torque performance than the conventional magnetic circuit structure without increasing the manufacturing cost.
Still another object of the present invention is to provide a magnetic circuit structure that can improve the torque performance when the magnetomotive force is large.